According to an art related to the invention, exhaust gas purification means is installed in an internal combustion engine. To cause the exhaust gas purification means to function and purify exhaust gas, for example, as in the case where exhaust gas is purified by reducing NOx by means of an NOx occlusion-reduction type catalyst, an addition agent composed of a reducing agent such as fuel is required. As one of such methods of exhaust gas purification, there is a method in which fuel is injected into an exhaust pipe of an internal combustion engine. Arts for causing exhaust gas purification means to function by adding fuel to exhaust gas are disclosed, for example, in Japanese Patent Application Laid-Open No. 6-108829, Japanese Patent Application Laid-Open No. 10-141048, and Japanese Patent Application Laid-Open No. 5-302509.
The arts disclosed in the above-mentioned publications, namely, Japanese Patent Application Laid-Open No. 6-108829, Japanese Patent Application Laid-Open No. 10-141048, and Japanese Patent Application Laid-Open No. 5 -302509 relate to the addition of light oil as fuel with the intention of purifying exhaust gas by means of exhaust gas purification means, and to optimizing control thereof.
Addition agent supply means for adding an addition agent is installed in an exhaust pipe of an internal combustion engine. Hence, the addition agent supply means is exposed to exhaust gas that contains unburnt components such as carbon particulates, particulate matters generated from sliding portions of the internal combustion engine, or the like. As a result, there is an apprehension, for example, that foreign matters will be stuck in a supply port of the addition agent supply means or that open-close control of the supply port will be adversely affected. In such a case, the addition agent supply means continues to supply exhaust gas with a reducing agent. Thus, if fuel such as light oil is used as the reducing agent, fuel burns in the exhaust gas purification means and the temperature thereof tends to become extraordinarily high. If the temperature of the exhaust gas purification means rises, a problem such as a deterioration in performance of the exhaust gas purification means is caused.
On the other hand, there is an apprehension that soot, viscous components, and the like contained in exhaust gas will adhere to the supply port of the addition agent supply means. In this case, the supply port of the addition agent supply means is clogged, and it becomes difficult to supply exhaust gas with fuel. As a result, the exhaust gas purification means may not function sufficiently. That is, if an NOx reduction catalyst is used as an example of the exhaust gas purification means, the air-fuel ratio of exhaust gas cannot be shifted toward the rich side. Hence, it becomes difficult to reduce NOx. This causes a problem in that the amount of NOx to be discharged cannot be reduced. If a particulate filter for collecting particulate matters contained in exhaust gas is used as an example of the exhaust gas purification means, the temperature of the filter cannot be raised. Thus, combustion of the collected particulate matters becomes insufficient, and the particulate matters are deposited in the filter. This causes a problem in that driveability and fuel consumption deteriorate due to an increase in pressure loss.